THE GROWTH OF GROUPS 93 



it, for the following reason : Some groups, such as those 

 mentioned, are marked off by decided characters, but 

 others are less clearly marked, so that the question would 

 often arise in reference to a particular group, " Ought 

 this to be included in our estimate ? " We might indeed 

 make the number appear large or small according to our 

 personal inclination. The same difficulty is often felt by 

 the taxonomist in regard to certain groups. The same 

 question arises, " Ought they to be regarded as species 

 or overlooked ? " 



Since the object of this book is to suggest rather than 

 to prove, little is to be gained by describing many cases, 

 but three others will be mentioned in this chapter as 

 illustrating further the occurrence of characters and 

 groups. 



The first of these illustrates the occurrence of a dis- 

 tinct character which was often found singly, and on one 

 occasion in a group. 



The presence of a white line on the breast between the 

 fore-legs is not uncommon in rats and other mammals. 

 The fact first came to notice in Madras city, where on 

 one occasion, two out of six hundred house rats were 

 marked in this manner. The mark was not quite alike 

 in both, being longer in one, so as to extend to the 

 abdomen. There was no reason for supposing that both 

 had been taken from the same house, although they 

 might have been. We may be sure that they, or a near 

 ancestor marked in a similar fashion, were born from 

 normal parents. No other explanation of the occurrence 

 seems possible. There is no doubt that the animals 

 marked in this fashion arose in some way from the 

 normal stock, since they differ only in the one character. 



